Bellatrix remembers what she had said to Cissy when they were at Spinner's End.
"You should be proud. If I had sons, I would be happy to give them up to the service of the Dark Lord."
But now it seems, Bellatrix has changed her mind. She stares at her own son, a newborn baby sleeping softly in the crib. Rigel.
The Dark Lord had been reborn only 2 years ago, and he had already placed a death sentence on her nephew's head. Poor Draco. He was at Hogwarts, practicing so they could come and help him fulfill his task.
She had been pregnant with her own son, happy that the Dark Lord would soon have another soul to serve him. But now as she gazes upon him, sleeping innocently, she can't give him to the Dark Lord. He won't suffer like Draco.
She stands over him, her wand pointing at him, she cries. Tears roll down her cheeks and her lip trembles.
The baby begins to cry. And Bellatrix cries harder.
"Ssh." She calms herself. "Hush." She touches the child's head, stroking what little hair he has. "Hush little baby, don't say a word." She begins to sing. She chokes on several words but the baby is silenced a little by the comfort of his mother's voice.
"Avada Kedavra." She whispers, more tears staining her cheeks. A flash of green light illuminates the room and the child is silenced.
Bellatrix collapses next to the crib and sobs loudly. She doesn't know what she did, or how to explain it. How would she tell the Dark Lord that the child she promised to hand over to him is dead? How would she tell her husband that she murdered the son he'd longed for?
She cries harder if possible. Feeling empty and alone.